Polyethylene films are widely used in flexible packaging, such as heavy duty shipping sacks, stand-up pouches, detergent pouches, satchets, etc. A number of properties must be provided in order to meet the package requirements in terms of integrity and attractiveness. These properties include: 1) excellent optical properties, such as high gloss, high clarity and low haze, 2) sufficient abuse resistance, such as high tensile strength, high puncture and impact resistance, 3) good sealing properties, such as low seal initiation temperature, broad sealing window, high seal strength and high hot tack.
Conventional blown or cast polyethylene films were widely used in flexible packaging, either as stand-alone packaging or lamination film. With market trends towards sustainability, flexible packaging continues to be downgauged. As a result, higher film stiffness combined with toughness is required. For normal blown or cast polyethylene (or “PE”) films, the 2% secant tensile modulus is preferably about 150˜250 MPa. By involving middle density or high density PE, stiffness can be enhanced, but that will compromise toughness and film clarity.
Bi-orientation is one of the popular fabrication processes to boost film stiffness and toughness. Clarity, stiffness, and toughness of the film can be dramatically improved by orientation. However, shrinkage properties are also gained through orientation. For most flexible packaging, residual shrinkage is not a desired property because high residual shrinkage will lead to sealing winkles, dimensional instability or other problems in downstream lamination or printing processes. Most of the incumbent polyethylene orientation technologies, like the double bubble process, are designed to produce a shrink film, not a dimensionally stable base film, such as lamination film for flexible packaging.
U.S. Pat. Nos. 4,354,997 and 4,463,153 disclose a process for producing biaxially oriented blown polyethylene films by biaxially stretching non-stretched tubular films produced from an ethylene polymer, such as low density polyethylene, linear low density polyethylene or a copolymer of ethylene as a main component and an alpha-olefinically unsaturated monomer. The non-stretched tubular film is expanded and extended under particular conditions at a special temperature profile simultaneously in the cross-direction and in the machine direction by means of internal air pressure.
European Patent Application 0 240 705 relates to tubular heat shrinkable, biaxially stretched, blown films of a mixture of (A) 90 to 50 percent by weight of a linear ethylene/alpha-olefin copolymer having a density, of 0.90 to 0.93 g/cm3 at 25° C. and (B) 10 to 50 percent by weight of an ethylene polymer having a density of 0.87 to 0.91 g/cm3 at 25° C. and less than the density of the copolymer (A) by at least 0.014 g/cm3. The non-stretched tubular film is expanded and stretched and, at the same time, biaxially oriented simultaneously in the cross-direction and machine direction by means of air pressure in the tube.
British Patent specification 866,820 relates to films made of high or low pressure polyethylene, polypropylene or ethylene/propylene copolymers. The polymeric material which has been oriented by uni-axial or bi-axial stretching during its production is first subjected to high energy ionizing irradiation. Then the material is heated and uni- or bi-axially oriented and finally cooled. A tubular film is used as a starting material for the first biaxial orientation. Due to the irradiation, the high temperature tensile strength of the film is increased. The biaxial orientation after irradiation is carried out by inflating the film with air or other gas or with a liquid to form a bubble. According to this bubble technique a blown film is produced which is simultaneously oriented in the cross-direction and in the machine direction.
U.S. Pat. No. 4,680,207 relates to a biaxially oriented linear low density polyethylene film which has been stretched in the cross-direction at a stretching ratio of greater than 1 to less than 3 and in the machine direction at a stretching ratio of less than 6 but greater than the stretching ratio in the cross-direction. The film is produced as follows: a molten linear low density polyethylene which is optionally mixed with a low density non-linear polyethylene resin is extruded, blown to a film and biaxially oriented in special equipment. The film is useful for producing tubular and heavy duty shipping sacks.
For producing shrinkable films having high optical clarity, good shrink properties and good mechanical properties, British Patent specification 2,097,324 suggests a film made by stretching a film made of the following homogeneous polymeric composition:
(1) 5 to 100 weight percent of a linear copolymer of ethylene with at least one C.sub.8-C.sub.18-alpha-olefin, said polymer having a density of 0.900 to 0.940 g/cm3 and two distinct crystallite melting regions below 128° C. as determined by differential scanning calorimetry (DSC), the temperature difference between those regions being at least 15° C. and(2) 0 to 95 weight percent of at least one polymer selected from the group consisting of ethylene homopolymers and copolymers of ethylene with an ethylenically unsaturated comonomer, said polymer having only one crystallite melting point below 128° C.The stretching of the film must be carried out within the temperature range defined by the two crystallite melting points of the copolymer (1). The polymers (2) are conventional ethylene homopolymers or copolymers. According to British Patent Specification 2,097,324 “conventional ethylene homopolymers or copolymers” are high density or low density polyethylene made at high pressure or low pressure. According to British Patent Specification 2,097,324, such conventional ethylene polymers having only one crystallite melting point below 128° C., are not useful for producing the shrink films and linear copolymers of ethylene and 1-octene or another alpha-olefin wherein the alpha-olefin comonomer is present in such small amounts that a second DSC peak is not observed are not suitable for producing the shrink films either. The films are produced on a tubular film production line which combines the extrusion and the orientation of the film. British Patent 2,097,324 teaches by reference to U.S. Pat. No. 3,141,912 that the tubular film is oriented by a combination of expansion and longitudinal stretching. The film is biaxially oriented in a continuous manner in two mutually perpendicular directions.
The ethylene polymers and polymer compositions disclosed in U.S. Pat. Nos. 4,354,997, 4,463,153 and 4,680,207, in European Patent Application 0 240 705 and in British Patent specifications 866,820 and 2,097,324 are useful for producing films which are simultaneously bioriented in the machine direction and cross-direction. However, only blown tubular films can be bioriented according to the teaching in the above-mentioned publications. A special processing line is required for the biorientation process. The processing line must be able to maintain a permanent gas pressure inside the tubular film. The teaching of the above-mentioned publications is not useful for biorienting cast films or other flat, i.e. non-tubular films.
Flat films such as cast films are usually bioriented in at least two steps, first in one direction and then in the other direction. Prior attempts to biorient, on a cast film production line, a flat film produced from the polymers disclosed in British Patent Specification 2,097,324 were not successful, as reported in U.S. Pat. No. 5,589,561.
European Patent Application 0212731 suggests the preparation of a mono-axially oriented cast film on the basis of an ethylene homo- or copolymer having a density below 940 kg/m3. The ethylene polymer may be blended with high density polyethylene or polypropylene and/or different types of ethylene homo or copolymers having a low density. The use of low density ethylene homopolymers, optionally mixed with minor amounts of other ethylene polymers, for preparing the mono-axially oriented film is exemplified.
However, it has been observed that polyethylene (PE) orientation is very difficult in both flat cast tenter frame and blown double bubble processes due to the narrow orientation window of PE. For the double-bubble process, film converters typically either co-extrude PE with polypropylene (PP) or crosslink PE to facilitate the second bubble stability. For the tenter-frame process, PP is widely used and known as biaxially oriented PP (BOPP) film in the packaging industry, while PE is not commercially fabricated by this process due to its narrow orientation window.
It would be desirable to provide a new bioriented film based on ethylene polymers which can be produced from a process involving at least two sequential orientation steps, such as by using the tenter frame process in which the processing conditions, such as temperature and stretching ratio, can be chosen within a relatively broad range and the process parameters may be changed to some extent during the stretching process without a substantial loss in quality of the bioriented film.
Accordingly, in one aspect, the present invention is a process for forming a biaxially oriented film comprising first selecting a polyolefin resin wherein said polyolefin resin comprises a linear low density polyethylene resin characterized by having from 9 to 35 weight percent of the total amount of the linear low density polyethylene resin eluting from a crystallization elution fractionation (CEF) instrument (at a temperature greater than 97.0° C.; and further characterized by having a comonomer distribution ratio (CDR) of from 33 to 80 and a molecular weight ratio (Mw Ratio) of from 0.15 to 0.45. Next, a film is formed from the polyolefin resin selected in the first step. Finally the film is oriented, preferably in a range of from 3 to 5 times in the machine direction and from 3 to 7 times in the cross direction, such that the resulting film is characterized by having: a) an ultimate elongation at least 1.5 times greater in the machine direction (MD) as compared to the cross direction (CD), b) a 2% secant modulus which is a least 1.25 times greater in the CD as compared to the MD, and c) free residual shrinkage of less than 10% in the MD and less than 10% in the CD when exposed to a temperature of 90° C. for 10 minutes, more preferably <5% in both the MD and CD.
In another aspect, the present invention is a film comprising a polyolefin resin comprising a linear low density polyethylene resin characterized by having from 9 to 35 weight percent of the total amount of linear low density polyethylene resin elutes at a temperature greater than 97.0° C. in CEF; and further characterized by having a CDR of from 33 to 80 and a Mw Ratio of from 0.15 to 0.45, wherein said film is characterized by having: a) an ultimate elongation at least 1.5 times greater in the MD as compared to the CD, b) a 2% secant modulus which is at least 1.25 times greater in the CD as compared to the MD, and c) free residual shrinkage of less than 10% in the MD and less than 10% in the CD when exposed to a temperature of 90° C. for 10 minutes, more preferably <5% in both the MD and CD.